The exchange of heat from a hot gas at atmospheric pressure to pressurized air may be performed in a recuperator, of which many conventional designs are available. These commercial designs are limited in size and have a poor service history when applied to large heat recovery applications, such as recovery of waste heat from the exhaust gas stream of a utility size combustion turbine. Waste heat from a combustion turbine may be used to heat compressed air stored for power generation purposes in compressed air energy storage (CAES) plants, or other process requiring heated compressed air.
CAES systems store energy by means of compressed air in a cavern during off-peak periods. Electrical energy is produced on-peak by admitting compressed air from the cavern to one or several turbines via a recuperator. The power train comprises at least one combustion chamber heating the compressed air to an appropriate temperature. To cover energy demands on-peak a CAES unit might be started several times per week. To meet load demands, fast start-up capability of the power train is mandatory in order to meet requirements of the power supply market. However, fast load ramps during start-up impose thermal stresses on the power train by thermal transients. This can have an impact on the lifetime of the power trains in that lifetime consumption increases with increasing thermal transients. For these types of applications, the physical size of the heat exchanger and the large transient thermal stresses associated with rapid heating of the recuperator during startup have proven to be beyond the capability of conventional recuperator equipment.
Common to all heat recovery air recuperators (HRARs), the temperature of the exhaust-gas stream declines from the exhaust-gas inlet to the exhaust-gas outlet of the heat exchanger. The amount of heat transferred in each heat exchanger tube row over which the exhaust-gas flows is proportional to the temperature difference between the exhaust-gas and the fluid in the heat exchanger tubes. Therefore, for each successive row of heat exchanger tubes in the direction of exhaust-gas flow, a smaller amount of heat is transferred, and the heat flux from the exhaust-gas to the fluid (e.g., compressed air) inside the tube declines with each tube row from the inlet to the outlet of the heat exchanger section. Therefore, for each successive row of heat exchanger tubes in the direction of gas flow, the temperature of the tube metal is determined by both the amount of heat flux across the tube wall and the average temperature of the fluid inside the tube.
For example, in a conventional recuperator, the temperature of the heat exchanger tube metal is determined by both the amount of heat flux across the heat exchanger tube wall and the average temperature of the flow medium inside the heat exchanger tube. Since the heat flux declines from the inlet to the outlet of the recuperator section, the temperature of the heat exchanger tube metal is different for each row of heat exchanger tubes included in the recuperator section.
Each manifold (header) of a horizontal heat recovery air recuperator (HRAR) that runs perpendicular to the exhaust-gas flow acts as a collection point for multiple rows of tubes. These headers are of relatively large diameter and thickness to accommodate the multiple tube rows. FIGS. 1a and 1b are two views of such an assembly 100, known as a multi-row header-and-tube assembly, utilized in typical heat exchanger arrangements. Included in the assembly 100 is a header 101 and multiple tube rows 105A-105C. As shown in FIG. 1a, each individual tube row 105A-105C includes multiple tubes. In the interest of clarity of illustration, FIG. 1b only shows a single tube in each tube row 105A-105C. Since each of tube rows 105A-105C is at a different temperature, the mechanical force due to thermal expansion is different for each tube row 105A-105C. Such differential thermal expansion causes stress at tube bends and the attachment point of each individual tube to the header 101. Further, also contributing to thermal stresses at the attachment point of each individual tube to the header 101 is a difference in thickness between the relatively thin-wall tubes as compared to the thick-wall header 101. Under certain operating conditions, these stresses can cause failure of the attachment point, especially if the assembly 100 is subjected to many cycles of heating and cooling. Accordingly, a need exists for a flexible recuperator for large-scale utility plant applications that is capable of both rapid heating and cooling as well as a large number of start-stop cycles.